Dress Down doesn't trigger properly against Dryad of the Ilysian Grove + Valakut, the Molten Pinnacle
My opponent had a Dryad of the Ilysian Grove and two Valakut, the Molten Pinnacle in play (without having actual Mountains). They put two Valakut triggers on the stack and I cast a Dress Down. Dress Down resolves, but their lands still have all the basic lands types and they are able to kill my creatures, without any sense.
This is correct. Dress down removes abilities in layer 6 AFTER dryad changes the land types in layer 4. So the dryad having abilities or not doesn't change the land types. Even with dress down in play, all of their lands should be all basic types. See comprehensive rules 613. And timestamps are irrelevant since these effects are happening in different layers (which means this is the case even if dryad enters after the dress down)